Helical needle apparatus for creating a virtual electrode used for the ablation of tissue

ABSTRACT

A surgical apparatus for delivering a conductive fluid to a target site for subsequent formation of a virtual electrode to ablate bodily tissue at the target site by applying a current to the delivered conductive fluid. The surgical apparatus includes an elongated device forming a helical needle. The helical needle is configured to engage bodily tissue and is hollow for delivering conductive fluid from a fluid source. Finally, the helical needle terminates in a needle tip. In one preferred embodiment, an electrode is associated with the helical needle for applying a current to conductive fluid delivered from the helical needle. During use, following delivery of conductive fluid, the electrode applies a current to the delivered conductive fluid for creating a virtual electrode. The virtual electrode ablates bodily tissue contacted by the conductive fluid.

This application claims the benefit of U.S. Provisional Application No.60/091,969, filed on Jul. 7, 1998.

FIELD OF THE INVENTION

The present invention relates generally to an apparatus for creating avirtual electrode. More particularly, the present invention relates toan apparatus for the creation of a virtual electrode that is useful forthe ablation of soft tissue and neoplasms.

BACKGROUND OF THE PRESENT INVENTION

The utilization of an electric current to produce an ameliorative effecton a bodily tissue has a long history, reportedly extending back to theancient Greeks. The effects on bodily tissue from an applied electriccurrent, and thus the dividing line between harmful and curativeeffects, will vary depending upon the voltage levels, current levels,the length of time the current is applied, and the tissue involved. Onesuch effect resulting from the passage of an electric current throughtissue is heat generation.

Body tissue, like all non-superconducting materials, conducts currentwith some degree of resistance. This resistance creates localizedheating of the tissue through which the current is being conducted. Theamount of heat generated will vary with the power P deposited in thetissue, which is a function of the product of the square of the currentI and the resistance R of the tissue to the passage of the currentthrough it (P=I²R.).

As current is applied to tissue, then, heat is generated due to theinherent resistance of the tissue. Deleterious effects in the cellsmaking up the tissue begin to occur at about 42° Celsius. As thetemperature of the tissue increases due to heat generated by thetissue's resistance, the tissue will undergo profound changes andeventually, as the temperature becomes high enough, that is, generallygreater than 45° C., the cells will die. The zone of cell death is knownas a lesion and the procedure followed to create the lesion is commonlycalled an ablation. As the temperature increases beyond cell deathtemperature, complete disintegration of the cell walls and cells causedby boiling off of the tissue's water can occur. Cell death temperaturescan vary somewhat with the type of tissue to which the power is beingapplied, but generally will begin to occur within the range of 45° to60° C., though actual cell death of certain tissue cells may occur at ahigher temperature.

In recent times, electric current has found advantageous use in surgery,with the development of a variety of surgical instruments for cuttingtissue or for coagulating blood. Still more recently, the use ofalternating electric current to ablate, that is, kill, various tissueshas been explored. Typically, current having a frequency from about 3kilohertz to about 300 gigahertz, which is generally known asradiofrequency or radiofrequency (RF) current, is used for thisprocedure. Destruction, that is, killing, of tissue using an RF currentis commonly known as radiofrequency ablation. Often radiofrequencyablation is performed as a minimally invasive procedure and is thusknown as radiofrequency catheter ablation because the procedure isperformed through and with the use of a catheter. By way of example,radiofrequency catheter ablation has been used to ablate cardiac tissueresponsible for irregular heart beats or arrythniias.

The prior art applications of current to tissue have typically involvedapplying the current using a “dry” electrode. That is, a metal electrodeis applied to the tissue desired to be affected and a generated electriccurrent is passed through the electrode to the tissue. A commonly knownexample of an instrument having such an operating characteristic is anelectrosurgical instrument known as a “bovie” knife. This instrumentincludes a cutting/coagulating blade electrically attached to a currentgenerator. The blade is applied to the tissue of a patient and thecurrent passes through the blade into the tissue and through thepatient's body to a metal base electrode or ground plate usually placedunderneath and in electrical contact with the patient. The baseelectrode is in turn electrically connected to the current generator soas to provide a complete circuit.

As the current from the bovie knife passes from the blade into thetissue, the resistance provided by the tissue creates heat. In thecutting mode, a sufficient application of power through the bovie knifeto the tissue causes the fluid within the cell to turn to steam,creating a sufficient overpressure so as to burst the cell walls. Thecells then dry up, desiccate, and carbonize, resulting in localizedshrinking and an opening in the tissue. Alternatively, the bovie knifecan be applied to bleeding vessels to heat and coagulate the bloodflowing therefrom and thus stop the bleeding.

As previously noted, another use for electrical instruments in thetreatment of the body is in the ablation of tissue. To expand further onthe brief description given earlier of the ablation of cardiac tissue,it has long been known that a certain kind of heart tissue known assino-atrial and atrio-ventricular nodes spontaneously generate anelectrical signal that is propagated throughout the heart alongconductive pathways to cause it to beat. Occasionally, certain hearttissue will “misfire,” causing the heart to beat irregularly. If theerrant electrical pathways can be determined, the tissue pathways can beablated and the irregular heartbeat remedied. In such a procedure, anelectrode is placed via a catheter into contact with the tissue and thencurrent is applied to the tissue via the electrode from a generator ofRF current. The applied current will cause the tissue in contact withthe electrode to heat. Power will continue to be applied until thetissue reaches a temperature where the heart tissue dies, therebydestroying the errant electrical pathway and the cause of the irregularheartbeat.

Another procedure using RF ablation is transurethral needle ablation, orTUNA, which is used to create a lesion in the prostate gland for thetreatment of benign prostatic hypertrophy (BPH) or the enlargement ofthe prostate gland. In a TUNA procedure, a needle having an exposedconductive tip is inserted into the prostate gland and current isapplied to the prostate gland via the needle. As noted previously, thetissue of the prostate gland heats locally surrounding the needle tip asthe current passes from the needle to the base electrode. A lesion iscreated as the tissue heats and the destroyed cells may be reabsorbed bythe body, infiltrated with scar tissue, or just become non-functional.

While there are advantages and uses for such “dry” electrodeinstruments, there are also several notable disadvantages. One of thesedisadvantages is that during a procedure, coagulum-dried blood cells andtissue cells-will form on the electrode engaging the tissue. Coagulumacts as an insulator and effectively functions to prevent currenttransfer from the blade to the tissue. This coagulum “insulation” can beovercome with more voltage so as to keep the current flowing, but onlyat the risk of arcing and injuring the patient. Thus, during surgerywhen the tissue is cut with an electrosurgical scalpel, a build-up ofcoagulated blood

A typical lesion created with a dry electrode using RF current and asingle insertion will normally not exceed one centimeter in diameter.This small size—often too small to be of much or any therapeuticbenefit—stems from the fact that the tissue surrounding the needleelectrode tends to desiccate as the temperature of the tissue increases,leading to the creation of a high resistance to the further passage ofcurrent from the needle electrode into the tissue, all as previouslynoted with regard to the formation of coagulum on an electrosurgicalscalpel. This high resistance—more properly termed impedance sincetypically an alternating current is being used—between the needleelectrode and the base electrode is commonly measured by the RF currentgenerator. When the measured impedance reaches a pre-determined level,some prior art generators will discontinue current generation.Discontinuance of the ablation procedure under these circumstances isnecessary to avoid injury to the patient.

Thus, a typical procedure with a dry electrode may involve placing theneedle electrode at a first desired location; energizing the electrodeto ablate the tissue; continue applying current until the generatormeasures a high impedance and shuts down; moving the needle to a newlocation closely adjacent to the first location; and applying currentagain to the tissue through the needle electrode. This cycle ofelectrode placement, electrode energization, generator shut down,electrode re-emplacement, and electrode re-energization, will becontinued until a lesion of the desired size has been created. As noted,this increases the length of the procedure for the patient.Additionally, multiple insertions increases the risk of at least one ofthe placements being in the wrong location and, consequently, the riskthat healthy tissue may be undesirably affected while diseased tissuemay be left untreated. The traditional RF ablation procedure of using adry ablation therefore includes several patient risk factors that bothpatient and physician would prefer to reduce or eliminate.

The therapeutic advantages of RF current could be increased if a largerlesion could be created safely with a single positioning of thecurrent-supplying electrode. A single positioning would allow theprocedure to be carried out more expeditiously and more efficiently,reducing the time involved in the procedure. Larger lesions can becreated in at least two ways. First, simply continuing to apply currentto the patient with sufficiently increasing voltage to overcome theimpedance rises will create a larger lesion, though almost always withundesirable results to the patient. Second, a larger lesion can becreated if the current density, that is, the applied electrical energy,could be spread more efficiently throughout a larger volume of tissue.Spreading the current density over a larger tissue volume wouldcorrespondingly cause a larger volume of tissue to heat in the firstinstance. That is, by spreading the applied power throughout a largertissue volume, the tissue would heat more uniformly over a largervolume, which would help to reduce the likelihood of generator shutdowndue to high impedance conditions. The applied power, then, will causethe larger volume of tissue to be ablated safely, efficiently, andquickly.

Research conducted under the auspices of the assignee of the presentinvention has focused on spreading the current density throughout alarger tissue volume through the creation, maintenance, and control of a“virtual electrode” within or adjacent to the tissue to be ablated. Avirtual electrode can be created by the introduction of a conductivefluid, such as isotonic or hypertonic saline, into or onto the tissue tobe ablated. The conductive fluid will facilitate the spread of thecurrent density substantially equally throughout the extent of the flowof the conductive fluid, thus creating an electrode—a virtualelectrode—substantially equal in extent to the size of the deliveredconductive fluid. RF current can then be passed through the virtualelectrode into the tissue.

A virtual electrode can be substantially larger in volume than theneedle tip electrode typically used in RF interstitial ablationprocedures and thus can create a larger lesion than can a dry, needletip electrode. That is, the virtual electrode spreads or conducts the RFcurrent density outward from the RF current source—such as a currentcarrying needle, forceps or other current delivery device—into or onto alarger volume of tissue than is possible with instruments that rely onthe use of a dry electrode. Stated otherwise, the creation of thevirtual electrode enables the current to flow with reduced resistance orimpedance throughout a larger volume of tissue, thus spreading theresistive heating created by the current flow through a larger volume oftissue and thereby creating a larger lesion than could otherwise becreated with a dry electrode.

While the efficacy of RF current ablation techniques using a virtualelectrode has been demonstrated in several studies, the currentlyavailable instruments useful in such procedures lags behind the researchinto and development of hoped-for useful treatment modalities for theablation of soft tissue and malignancies.

It would be desirable to have an apparatus capable of creating a virtualelectrode for the controlled application of tissue ablating RF electriccurrent to a tissue of interest so as to produce a lesion of desiredsize and configuration.

SUMMARY OF THE INVENTION

One aspect of the present invention provides a surgical apparatus fordelivering conductive fluid to a target site for subsequent formation ofa virtual electrode to ablate bodily tissue at the target site byapplying a current to the delivered conductive fluid. The surgicalapparatus comprises an elongated device forming a helical needleassembly. The helical needle assembly includes a helical needleconfigured to engage bodily tissue. The helical needle is hollow fordelivering conductive fluid from a fluid source and forms a needle tip.In one preferred embodiment, an electrode is associated with the helicalneedle assembly for applying a current to conductive fluid deliveredfrom the helical needle assembly. During use, the helical needleassembly is maneuvered into contact with bodily tissue at a desiredlocation. Conductive fluid is delivered to the tissue via the hollowhelical needle. The electrode applies a current to the so-deliveredconductive fluid, thereby creating a virtual electrode for ablating thebodily tissue.

Another aspect of the present invention relates to a surgical system forcreating a virtual electrode to ablate bodily tissue. The systemincludes a fluid source, a current source and a surgical instrument. Thefluid source maintains a supply of conductive fluid. The current sourceis configured to selectively supply a current. Finally, the surgicalinstrument includes an elongated device forming a helical needle and anelectrode associated with the helical needle. The helical needle isconfigured to engage bodily tissue. Further, the helical needle ishollow and is fluidly connected to the fluid source for delivering theconductive fluid. Finally, the helical needle terminates in a needletip. The electrode is associated with the helical needle and isconnected to the current source. With this configuration, during use,the helical needle is maneuvered into engagement with a desired locationof bodily tissue. Conductive fluid is delivered to the bodily tissue viathe helical needle. The current source is then activated to supply acurrent to the electrode, in turn applying a current to the conductivefluid delivered from the helical needle. Application of the current tothe delivered conductive fluid creates a virtual electrode, therebyablating bodily tissue in contact therewith.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a surgical system for creating a virtualelectrode to ablate bodily tissue in accordance with the presentinvention;

FIG. 2 is an enlarged, side view of a helical needle portion of asurgical instrument used with the system of FIG. 1;

FIG. 3 is a side view of an alternative helical needle in accordancewith the present invention;

FIG. 4 is an enlarged, side view of an alternative helical needle inaccordance with the present invention;

FIG. 5 is an enlarged, side view of an alternative helical needle inaccordance with the present invention;

FIG. 6 is an enlarged, side view of an alternative helical needle inaccordance with the present invention;

FIG. 7 is an enlarged, exploded view of an alternative helical needleassembly in accordance with the present invention;

FIG. 8 is an enlarged, side view of an alternative helical needleassembly in accordance with the present invention;

FIG. 9 is an enlarged, side view of an alternative helical needleassembly in accordance with the present invention;

FIG. 10 is an enlarged, perspective view of an alternative helicalneedle in accordance with the present invention;

FIG. 11 is an enlarged, perspective view of an alternative helicalneedle assembly in accordance with the present invention;

FIG. 12 is an enlarged, cross-sectional view of an alternative helicalneedle in accordance with the present invention;

FIG. 13 is an enlarged, side view of an alternative helical needle inaccordance with the present invention;

FIG. 14 is an enlarged, cross-sectional view of an alternative helicalneedle in accordance with the present invention; and

FIG. 15 is an enlarged, perspective view of an alternative helicalneedle assembly in accordance with the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

FIG. 1 illustrates in block form a surgical system 20 for RF ablationuseful with the present invention. The surgical system 20 includes acurrent source of radiofrequency alternating electric current 22, afluid source of RF ablating fluid 24, including but not limited tosaline and other conductive solutions, and a surgical instrument 26 fordelivering the RF current and the ablation fluid to a tissue site (notshown) for ablation purposes. In one preferred embodiment, the surgicalinstrument 26 is connected to the current source 22 and the fluid source24. It will be understood that the current source 22 and the fluidsource 24 may be combined into a single operational structure controlledby an appropriate microprocessor for a controlled delivery of ablatingfluid and a controlled application of RF current, both based uponmeasured parameters such as but not limited to, flow rate, tissuetemperature at the ablation site and at areas surrounding the ablationsite, impedance, the rate of change of the impedance, the detection ofarcing between the surgical instrument and the tissue, the time periodduring which the ablation procedure has been operating, and additionalfactors as desired.

While the surgical instrument 26 is shown as being connected to both thecurrent source 22 and the fluid source 24, the present system is not solimited but could include separate instruments for those purposes. Forexample, a separate needle or similar apparatus could be used to deliverthe current and a separate needle or needles could be used to deliverfluid to the target tissue. In addition, the application of the surgicalsystem 20 illustrated in FIG. 1 is not limited to the use of straightneedles or helical needles as surgical instruments but could find usewith any type of instrument wherein a conductive solution is deliveredto a tissue and an RF current is applied to the tissue through theconductive fluid. Such instruments thus would include straight needles,helical needles, forceps, roller balls, or other instruments for thetreatment of vascular disorders, and any other instrument.

As described above, the surgical instrument 26 may assume a wide varietyof forms. In accordance with the present invention, however, thesurgical instrument includes an elongated device terminating in ahelical needle assembly configured to deliver the conductive fluid aswell as to apply a current to the so-delivered fluid. Variousembodiments of acceptable helical needle configurations are shown inFIGS. 2-15. For example, FIG. 2 illustrates one embodiment of a helicalneedle or helical needle assembly 30. The helical needle 30 ispreferably hollow, terminating in a needle tip 32 at a distal end 34thereof. The needle tip 32 defines an opening (not shown) for deliveringconductive fluid supplied to the helical needle via the fluid source 24(FIG. 1). Further, as previously described, the helical needle 30 ispreferably configured to serve as an electrode for applying a current,via the current source 22 (FIG. 1), to the delivered conductive fluid.With the above in mind, the helical needle defines a pitch P betweenadjacent coils 36 that increases in the distal direction. This varyingpitch facilitates first the engagement of the needle tip 32 with tissue(not shown) at a target site and its initial threading therein. As thepitch between adjacent coils 36 of the helical needle 30 decreases inthe proximal direction, the adjacent coils 36 more tightly engage thetissue, thereby providing a better seal between the tissue and theneedle coils 36. This greater sealing ability reduces the likelihoodthat the conductive fluid, which is heated during the ablation processto a temperature capable of killing cells, will not leak back along thetrack in the tissue made by the helical needle 30. Thus, the conductivesolution will tend to stay closely adjacent to the needle tip 32 ratherthan leak backwards along the needle track and unintentionally andundesirably damage or destroy other healthy tissue.

Referring now to FIGS. 3 and 4, alternative embodiments of a fluid andRF current delivery helical needle 40 and 50 are illustrated. Bothhelical needles 40 and 50 are hollow, terminating in a needle tip 42,52, respectively, and have a variable diameter that changes in theproximal to distal direction. In the case of the helical needle 40, anouter diameter of the helical needle 40 increases in the proximal todistal direction, whereas with the helical needle 50, the outer diameterdecreases in the proximal to distal direction. Varying the diameter inthis manner may allow for better sealing of the tissue (not shown) inthe affected region against the coils comprising the respective helicalneedle 40 or 50. That is, alternating the diameter in the manner shownincreases the loading of the tissue across the various coils unlike auniformly coiled needle, wherein the tissue loading is uniform acrossthe coils. Varying the diameter will also affect the amount of torquethat must be applied by a surgeon to screw the needle tip 42, 52 intothe tissue to be ablated. Thus, with respect to the helical needle 40,as the helical needle 40 is turned into the tissue, the torque necessaryto rotate the needle tip 42 increases because the coils force theengaged tissue into a smaller diameter, thus substantially sealing aresulting needle track against leakage of heated conductive fluid therealong. Similarly, with respect to the helical needle 50 as shown in FIG.4, the helical needle 50 first engages a relatively small portion oftissue and subsequently forces the larger diameter coils into the sameneedle track as followed by the initially small diameter needle tip 52.

Referring now to FIG. 5, an alternate embodiment of helical needle 60useful in an RF ablation procedure is shown. As with previousembodiments, the helical needle 60 is hollow to provide a flow path forconductive fluid, and defines a needle tip 62 through which theconductive fluid exits the helical needle 60. Additionally, the helicalneedle 60 has a diameter that first increases in the proximal to distaldirection and then decreases to a diameter somewhat similar to theinitial coil diameter at the needle tip 62.

Yet another alternative embodiment of a helical needle 70 is shown inFIG. 6. The helical needle 70 has a coil diameter that decreases in theproximal to distal direction for a predefined predetermined distance andthen increases to a diameter substantially equal to the originaldiameter. Once again, the helical needle 70 is hollow and has a needletip 72 defining an opening for the outflow of conductive fluid. Both ofthe embodiments 60 and 70 shown in FIGS. 5 and 6 provide for a varyingtorque and increased sealing ability due to the action of the helicalneedle 60 and 70 forcing the tissue (not shown) to follow the coilsthrough the tissue as the diameter thereof varies.

Yet another alternate embodiment of a helical needle assembly 80 isshown in exploded view in FIG. 7. The helical needle assembly 80comprises a plurality of concentric helical needles 82, 84 and 86.Alternatively, only two such helical needles could be provided oradditional helical needles may be used as desired. The use of aplurality of concentrically disposed helical needles 82, 84 and 86allows the physician to engage thinner tissues (not shown), such as theatrial wall. The concentric helical needles 82, 84 and 86 are preferablywound in the same direction to facilitate insertion and capture of thetissue. Each helical needle 82, 84 and 86 includes a needle tip 83, 85and 87, respectively. As desired, the helical needles 82, 84 and 86 arehollow to provide a fluid path for conductive fluid from the fluidsource 24 (FIG. 1) with the fluid exiting through needle tips 83, 85 and87 respectively. That is, one or more of the helical needles 82, 84 or86 could be used to provide fluid to the tissue to be ablated.Additionally, one or more of the helical needles 82, 84 or 86 may beused as a suction path for removal of conductive fluid.

Yet another alternative embodiment of a helical needle assembly 90 isdepicted in FIG. 8. In general terms, the helical needle assembly 90comprises a plurality of helical needles, here 92 and 94, wound parallelto one another. As with previous embodiments, each of the helicalneedles 92 and 94 are preferably hollow, terminating in an open, needletip 96 and 98, respectively. The use of a parallel assembly would enablethe physician performing an ablation procedure to use one of the helicalneedles 92 or 94 as a fluid path for providing conductive fluid from thefluid source 24 (FIG. 1) to the tissue (not shown) to be ablated and thesecond helical needle 92 or 94 as a vacuum source for removal of theconductive fluid. The use of suction to remove the ablation fluid eitherduring or at the end of the procedure will reduce the likelihood ofleakage of the hot conductive fluid backwards along the needle track.

Yet another alternative embodiment of a helical needle assembly 100 isshown in FIG. 9. The helical needle assembly 100 is similar to thehelical needle assembly 80 (FIG. 7) previously described. The helicalneedle assembly 100 includes outer and inner helical needles 102 and 104concentrically arranged. As with the embodiment shown in FIG. 7, theconcentric helical needles 102, 104 may be used to deliver fluid atdifferent depths in a tissue (not shown) or one or more flow paths couldbe used to provide suction and removal of the ablating solution from thetissue during or subsequent to the termination of the application of RFpower to the tissue, via the current source 22 (FIG. 1). As shown inFIG. 9, only two such concentric helical needles 102, 104 are shown,though multiple coils in excess of two could be used.

It will be understood that the various configurations could be combinedin several of the embodiments shown. For example, the variable pitchshown in FIG. 2 could be combined with the variable diameters shown inFIGS. 3-5 and 8. In addition, the variable diameter structure shown inFIGS. 3-6 could be combined with the parallel assembly constructionshown in FIG. 8.

Referring now to FIGS. 10 and 11, alternate embodiments of helicalneedle assemblies 110 and 120, including alternate means of anchoring aneedle to a tissue (not shown), are shown. With reference to FIG. 10,the helical needle assembly 110 is an archimedes type screw comprising acentral shaft 112 about which a helical flight 114 is wound. The centralshaft 112 is preferably hollow for delivering conductive fluid from thefluid source 24 (FIG. 1). The helical needle assembly 120 has asubstantially straight fluid and current delivery portion 122 mountedsubstantially centrally of a disc 124 having a plurality of startingthreads 126. The starting threads 126 will engage the tissue (notshown), such as a cardiac wall, and will anchor the helical needleassembly 120 thereto. Though it be understood that the straight needleportion 122 would penetrate the tissue wall to deliver fluid andelectric current from the fluid source 24 (FIG. 1) and the currentsource 22 (FIG. 1), respectively.

In the discussion of FIGS. 2-11, it has been understood that eachembodiment described would deliver conductive fluid through a singleaperture at the needle tip. The present invention is not so limitedhowever. Thus, the fluid could be delivered to the tissue (not shown) bylaser drilling a plurality of holes along the length of the variouscoiled needle configurations. These laser drilled fluid deliveryapertures could be configured as desired in terms of size and well asnumber and distance from each other along the longitudinal extent of thehelical needle. FIGS. 12-14 illustrate yet additional fluid deliverymethods. Thus, as shown at FIG. 12, a helical needle 130 could includeopenings 132 extending along part of an outer diameter of the helicalneedle 130. Alternatively, a helical needle 140 is shown in FIG. 13 asincluding an open slit 142 extending along an outer diameter thereof.Conversely, a helical needle 150 is depicted in FIG. 14 as having a coilstructure wherein a fluid delivery aperture or slit 152 extends alongthe inner diameter of the needle coils.

FIG. 15 illustrates yet another alternative embodiment of a helicalneedle assembly 160 in accordance with the present invention including apair of concentric helical needles 162 and 164. The first helical needle162 is a large diameter needle whose end 166 (shown partially) is usedto anchor the instrument in tissue 168. The second helical needle 164has a smaller diameter than the first helical needle 162 and is disposedsubstantially coaxially therewith. Each of the helical needles 162 and164 may include a hollow interior to provide a flow path for conductivefluid from the fluid source 24 (FIG. 1) and each could be electricallyactive, via the current source 22 (FIG. 1), through known appropriateconnections. Either or both of the helical needles 162, 164 may be usedto deliver conductive fluid and either of the helical needles 162, 164could be electrically active. Alternatively, the helical needles 162 and164 could form a bipolar needle wherein the helical needles 162, 164have opposing polarities and the current provided by one travels to theother.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A surgical apparatus for delivering conductive fluid to a target sitefor subsequent formation of a virtual electrode to ablate bodily tissueat the target site by applying a current to the delivered conductivefluid, the apparatus comprising: an elongated device forming a helicalneedle assembly including a first helical needle configured to engagebodily tissue, the first helical needle being hollow for deliveringconductive fluid from a fluid source and forming a needle tip.
 2. Thesurgical apparatus of claim 1, wherein the first helical needle forms anelectrode.
 3. The surgical apparatus of claim 1, wherein the firsthelical needle includes a plurality of adjacent coils, and furtherwherein a pitch between each of the plurality of adjacent coils isvariable.
 4. The surgical apparatus of claim 3, wherein the pitchdecreases proximally along at least a portion of the length of the firsthelical needle.
 5. The surgical apparatus of claim 1, wherein the firsthelical needle includes a plurality of adjacent coils defining an outerdiameter, and further wherein the outer diameter is variable along atleast a portion of a length of the first helical needle.
 6. The surgicalapparatus of claim 5, wherein the outer diameter decreases proximallyfrom the needle tip.
 7. The surgical apparatus of claim 5, wherein theouter diameter increases proximally from the needle tip.
 8. The surgicalapparatus of claim 5, wherein the first helical needle includes aproximal portion, a central portion and a distal portion, and furtherwherein the outer diameter decreases from the central portion to theproximal portion and the distal portion.
 9. The surgical apparatus ofclaim 5, wherein the first helical needle includes a proximal portion, acentral portion and a distal portion, and further wherein the outerdiameter increases from the central portion to the proximal portion andthe distal portion.
 10. The surgical apparatus of claim 1, wherein thehelical needle assembly further includes: a second helical needleconcentrically arranged with the first helical needle, the secondhelical needle having an outer diameter less than an inner diameterdefined by the first helical needle.
 11. The surgical apparatus of claim1, wherein the helical needle assembly further includes: a secondhelical needle wound parallel with the first helical needle.
 12. Thesurgical apparatus of claim 1, wherein the helical needle assemblyfurther includes: a second helical needle concentrically disposed withinthe first helical needle.
 13. The surgical apparatus of claim 1, whereinthe first helical needle includes: a central shaft, the shaft beinghollow for delivering conductive fluid; and a helical flight extendingfrom an outer circumference of the central shaft.
 14. The surgicalapparatus of claim 1, wherein the first helical needle includes: asubstantially straight tube, the tube being hollow for deliveringconductive fluid; a disc extending radially from the tube; and aplurality of starter threads extending from the disc spaced from thetube, the plurality of starter threads defining a helical pattern. 15.The surgical apparatus of claim 1, wherein the first helical needleforms openings along at least a portion of an outer diameter fordelivering conductive fluid.
 16. The surgical apparatus of claim 1,wherein the first helical needle forms a continuous slit along an outerdiameter for delivering conductive fluid.
 17. The surgical apparatus ofclaim 1, wherein the first helical needle forms a continuous slit alongan inner diameter for delivering conductive fluid.
 18. A surgical systemfor creating a virtual electrode to ablate bodily tissue, the systemcomprising: a fluid source maintaining a supply of conductive fluid; acurrent source for selectively supplying a current; and a surgicalinstrument including: an elongated device for forming a helical needleconfigured to engage bodily tissue, the helical needle being hollow andfluidly connected to the fluid source for delivering the conductivefluid and terminating in a needle tip, an electrode associated with thehelical needle, the electrode being connected to the current source forapplying a current to conductive fluid delivered from the helical needleto create a virtual electrode.
 19. The surgical system of claim 18,wherein the helical needle includes a plurality of adjacent coils, andfurther wherein a pitch between each of the plurality of adjacent coilsis variable.
 20. The surgical system of claim 18, wherein the helicalneedle forms openings along at least a portion of an outer diameter ofthe first helical needle for delivering conductive fluid.